Missing teeth disturb the perfect locking of dentition and the harmony of chewing. Neglecting these strongly influences somatic and mental harmony. Loss of molar teeth is most frequent which can be substituted initially by inserting bridgework. However, building in fixed bridgework can be impeded by loss of the posterior tooth supporting the row end. This problem can be solved only by removable prosthesis. Insufficient chewing due to lack of substitute teeth can adversely influence gastric and intestinal functions. Aesthetical look of visage is also altered, which can cause psychical disorders.
Several methods for substitution of teeth are known, including among others, methods providing special dental bridgework with fixed or with non-fixed end, the use of which can raise problems well known in the art.
Hungarian patent 210.237 granted on Nov. 1, 1994 relates to substitution of missing posterior teeth by metal bridgework provided with glass stump and a method to make the said metal bridgework. The dental bridge according to the above patent includes a conventional metal bridge body and one or more glass stump(s) attached to the metal bridge body with its (their) tapering end(s). The essence of the patent is, that the glass stump supporting the metal bridgework is formed from molten glass, and matches the shape of tooth saddle, and the glass stump, or, in case of several glass stumps, at least one of them is fastened to the terminal back element of the metal bridgework. When making the metal bridgework, a molten glass bar is pressed on the appropriate location of the model made by conventional method, followed by slowly cooling the glass mould corresponding to the shape of the saddle, after cooling, the sides opposite to the gums are ground and cut, the glass stump is temporarily fixed on the model and fixed in the metal bridgework previously prepared. A glass stump made from a glass crock by a German dentist already in 1936 was similar to the described means, but instead of melting, it was formed by grinding only.
According to the above referred Hungarian patent the glass bar was melted suitably at a temperature of 524 to 526.degree. C. The suitable glass composition disclosed in the description was the following: 73.0% SiO.sub.2, 7.0% B.sub.2 O.sub.3 /As.sub.2 O.sub.3 /Al.sub.2 O.sub.3, 2.5-5% MgO and the rest being ZnO/BaO/PbO/Fe.sub.2 O.sub.3 /Na.sub.2 O/K.sub.2 O.
The durability of special dental bridges with non-fixed end can substantially be increased by using support pillar (possibly support pillars) conforming to the shape of the gums. A great number of requirements can be set as to the proprieties of the fastening element of circular or elliptical cross-section shape. The most important requirements are as follows:
a) appropriate rigidity to endure strains resulting from chewing, long-term resistance against chemical reactions occurring in the mouth, PA1 b) long-term tissue tolerance and inactivity when used in human organism, PA1 c) appropriate softening characteristics for use in dental laboratory conditions, and subsequent plasticity so that its structure remains inactive, and its rigidity be maintained under chewing load as well.
According to experiences, particularly inorganic glasses meet these requirements. However, due to their composition, most glasses are not suitable for dental purposes, as they do not meet the above requirements. Some other glasses can be used only by compromises. Therefore, there is the need to provide a glass of special composition meeting most suitably the described conditions of use. It was the aim of the present invention to work out such a glass of special composition.
The so-called support pillar, provided as support of the free end of the dental bridgework must have a substantial mechanical resistance to endure the generally known high pressing and shearing forces occurring while chewing even when considering the altered distribution of stress compared to conventionally fixed dental bridgeworks. Unstrained silicate glass seems to be suitable for this purpose, its modulus of elasticity being usually in the interval from 500 to 800 mPa, and its compressive strength is high compared to conventional plastic materials.
The Mohs hardness of 5 to 7 of this glass is sufficiently high to make support pillars which would not be scratched by food. Namely, surface micro-scratches play an important role in breaking. Surface micro-scratches would deteriorate mechanical properties as well. Such glass shows high resistance to abrasion as well, although in the application according to the invention it is obviously not exposed to as intensive erosion as the teeth.
The thermal and electrical conductivity of glass is low. There is no danger that glass forms a galvanic cell in the mouth, and accelerates corrosion of parts made from other metals and built in the mouth.
Preferably the structural material to be inserted should possess chemical resistance. Silicate glasses usually meet this requirement. However, the composite reaction between glasses and solutions contacting the glasses such as saliva, should be considered. Such reactions can be basically separated in two processes being in interaction with each other. As a result of the first process, hydrogen ions are passing on the glass surface from the solution, penetrating by slow diffusion in the interior of the glass. Concurrently, alkaline ions, particularly sodium ions are passing in the solution in an amount equivalent with the hydrogen ions. This partial process makes the solution alkaline. The second process results in slow degradation of the polymer structure of glass during which all elements constituting the glass, go in solution. Both processes take place usually slowly at normal body temperatures, however, velocity can greatly alter according to the glass composition. In a disadvantageous case, a substantial local increase of pH and/or dissolving of toxic materials harmful for the organisation must be taken into account.
The selection of the structure material and the simplicity of the technology to be used, are also important factors, i.e. the support pillar could be formed and fastened in the bridge in customary dental mechanic's laboratories easily and in short time. From this point of view the dental glass pillar is particularly advantageous. The glass pillar of the invention can be simply heated up to the proper temperature, then pressed on the appropriate location of the plaster model, preferably using a tool designed for this purpose, and is cooled regularly to avoid unfavourable stresses. The above mentioned Hungarian patent does not meet all the above requirements.